The present invention relates to a developing device for developing an electrostatic latent image with a two-component developing agent and, more particularly, to a developing device which has two developing rollers and which can be suitably used in a two-color copying machine.
In a conventional two-color copying machine of this type, a plurality of cartridges for different colors are prepared. Each cartridge integrally has a developing device and a photosensitive drum. An operator selectively uses a cartridge of a desired color every time copying is performed. Cartridge changing is time-consuming and cumbersome, resulting in inconvenience.
The present inventors have developed a two-color developing device wherein a latent image on a photosensitive drum is developed by selectively operating first and second developing rollers upon depression of a button.
Demand has also arisen for a compact high-speed copying machine. Along with this, a compact photosensitive drum having a diameter of 80 mm or less has been devised.
A conventional two-color developing device uses two developing rollers and requires a large space. However, in order to guarantee a developing time at a high copying speed (30 sheets/minute), the diameter of the developing rollers cannotd be decreased. As a result, a compact high-speed two-color copying machine cannot be obtained.